validate email field on oracle database

You can validate the domain name portion of the email address using a DNS Lookup. I did this many years ago using a component purchased from MelissaData.

No Easy Way
I know of no way to validate individual email addresses without sending them an actual email message ("Click here if you want to stay on our emailing list").

Bouncebacks indicate that the mailbox was unavailable. It is not foolproof because their mailbox server could have been offline and some companies have turned off the bounceback capability of their mail server.

If you send them a message, you could put a reference to a uniquely named image (one for each email address) in each email message and mine the W3SVC logs to record attempts to load those uniquely named images. The image does not have to exist on your server. This is not foolproof because some people have image loading turned off in their browsers and some people delete email messages without reading them. Check with your company's attorney before doing this as some companies have strict policies on email messages sent from the company to customers.